Cryogenically cooled drill

ABSTRACT

This discloses a drilling unit in which the drill bit mechanism is placed at the end of the rotor shaft of a motor either by direct attachment to the shaft or by gear. Cryogenic cooling is provided by sending cryogenic fluid into the rear portion of the motor and causing it to flow toward the front end of the motor for cooling the motor, effluent from the motor being conveyed into and through the drill bit and through bearings and gearing, for cooling and cleansing. The coolant effluent from the drill bit mechanism and motor passes into the bore hole thereby cooling it and carrying cuttings and debris upwardly through the hole.

United States Patent Andrews et al.

[45] Mar. 21, 1972 [541 CRYOGENICALLY COOLED DRILL 3,424,254 1/1969 Huff..175/17 [72] Inventors: Donald W. Andrews, Eldorado Hills; Eu-

g i g L. Kessler; Roland A. Newton both p f Sacramento an of Cahf'Primary Examiner-James A. Leppink Assignee: A i Corporation. l Monte,Attorney-Edward o. Ansell, D. Gordon Angus and Donald Calif. 1 W. Graves22 Filed: July 31, 1969 [57] ABSTRACT N 4 478 [2 l 1 P 8 v Thisdiscloses a drilling unit in which the drill bit mechanism is placed atthe end of the rotor shaft ofa motor either by direct U.S. Cl tta h entto the haft o g'ean Cryogenic goofing is pro. [51] Int. Cl ..'...E2lb3/10 vided by Sending cryogenic d i the rear portion of the [58] Fleldof Search ..l75/ 17, 69, 71, 104, 107; motor and causing it to n towardthe from end of the motor 166/302 for cooling the motor, effluent fromthe motor being conveyed i into and through the drill bit and throughbearings and gear- [56] References C ted ing, for cooling and cleansing.The coolant effluent from the UNITED STATES PATENTS drill bit mechanismand motor passes into the bore hole 175/17 thereby coollnlg it andcarrying cuttlngs and debris upwardly 2, l l Mackay through the ho e2,355,342 8/1944 Van Wormen ..l75/l04 3,181,631 5/1965 Nielsen 175/10420 Claims, 3 Drawing Figures 42 /7 24 so 2267 /Z8 45 /6 46 34/ /6 23 25m 5o /s 47 5 /Z [4 a, 461 52 2s 3/ Z 2] /o/ 35 CRYOGENICALLY COOLEDDRILL This invention relates to earth drilling equipment and method ofdrilling, and has for an object to increase the speed or penetrationrate and the directional accuracy with which very long or deep holes arebored.

The invention is applicable to oil, water and gas well drilling, rockboring for shot holes, seismic bore holes and the like. The termearth-drilling as used herein means drilling through anything found inor on the earth, including drilling under water.

It is well known that the speed and efficiency of earth drilling isimproved by cooling the drill bits, and coolants and refrigerants haveheretofore been used with earth-drilling bits and equipment.

In accordance with the present invention, there is provided a moreefficient organization of cooled power drilling equipment than hasheretofore been known. The invention is carried out by use of a unitaryarrangement of a cryogenic-cooled motor suspended by cable or othersupport means at the work surface with means for transmitting the powerfrom the motor to the cutters, mounted in close proximity to the motor.Means is provided which conveys cryogenic fluid directly from the motorto the cutter mechanism from where it emerges into the borehole fromwhich it emerges with the cuttings.

A feature of the invention resides in provision for transmitting to themotor the cryogenic coolant, such as liquid nitrogen or other liquefiedgas. It is preferred that the electric supply cable to the motor shallbe immersed in the cryogenic supply fluid to reduce electricalresistivity of the cable.

According to other features, the cryogenic coolant is channeled throughand around the electric motor in a manner to reduce the temperature ofthe motor to or near the cryogenic fluid temperature. Thus theelectrical resistivity of the motor is reduced. The arrangement providesa motor of great power density located at the work area.

During motor operation, heat rejected by the motor is absorbed by thecryogenic fluid coolant. Effluent from the motor is routed through flowcircuitry from the motor to and through the drill cutter and bearings tocool and clean these parts. Effluent from the motor may be a liquid, agas, or a mixture of both.

Gaseous effluent emanating from the drill, in addition to maintainingthe cutting and the work surface at a very low temperature, clears thework area of cuttings and is the medium on which the cuttings arecarried out of the hole.

The foregoing and other features of the invention will be betterunderstood from the following detailed description and the accompanyingdrawing, of which:

FIG. 1 is a side view partially in cross section of a drill bit anddriving motor unit according to this invention;

FIG. 2 is an end view of the drill bit of FIG. 1; and

FIG. 3 is a side view, partially in cross section of another form ofdrill bit and driving motor unit according to this invention.

Referring to FIGS. 1 and 2 of the drawing, there is shown an electricmotor comprising a cylindrical housing 11 and a rotor 12 within thehousing. The rotor is shown as a cylindrical member rotatable within itshousing by ball bearings 13 at the forward end and a similar bearing 14at the rear end. A pipe nipple or the like 15 admits the electric cablecontaining the conductors for powering the motor. Since the internalelements by which the rotor of the motor is torqued are no part of thepresent invention, such elements are not shown.

At the forward end of the electric motor, and coaxial therewith, thereis fastened a drilling mechanism 16. This comprises a cutter cage 17comprising a cylindrical member 18 of somewhat greater diameter thanthat of the motor secured to the cylindrical motor housing 11 near theforward end of the motor housing by a ball bearing mechanism 19providing freedom of rotation between the motor housing and the cuttercage. The balls 20 of this ball bearing mechanism are held betweenconcentric ball races, the inner race 21 being securely fitted to themotor housing and the outer ball race 22 being securely fitted to theupper flange of the cutter cage.

A number of generally conical cutters 23, which may ordinarily be one,two or three in number (twobeing shown in the drawing), are rotatablymounted within the cutter cage with their forward cutting surfacesprotruding somewhat beyond the forward end 24 of the cage. These rotarycutters are mounted to the cage by ball bearing mechanisms25 eachcomprising a ball race 26 fastened in a cylindrical seat within thecutter cage and another ball race 27 fastened to a rearward hub 28 ofthe conical cutter, with provision for maintaining the balls 29-withinthe ball races. The axis of rotation of each rotary cutter extendsobliquely toward the, central longitudinal axis of the cutter cage andmotor, as shown. A hub- 30 between the cutting portion and the rearmosthub 28 of each cutter is provided with gear teeth 31 arranged concentricwith the axis of rotation of the respective cutter, All the cutters aredriven in rotation by bevel gear teeth 32 formed at the forward end 33of the rotor of the motor which protrudesforwardly from the rotorhousing within the cutter cage, and these bevel gear teeth mesh with thegear teeth 31 of the respective cutters.

The cutters will ordinarily have on their conical surfaces sharpenedspiral cutting ridges which are hard faced, for example with tungstencarbide or other sintered metal carbide. They may, if desired, be setwith industrial diamonds for very hard formation cutting. The cuttersare offset and overlapping to cut uniformly over the bottom of the hole.When a plurality of cutters are used in a cutter cage they should becircumferentially equally spaced. The cutters may be positioned so thattheir cutting surface is flat and horizontal, or it may be shaped asdesired.

The drilling unit is provided with a cooling system for operation bycryogenic fluid. This comprises a passageway 34 within the wall ofcylindrical housing 11 of the motor, this passageway extending spirallyaround the rotor and within the housing wall. The fluid inlet of thispassageway is connected to a suitable pipe fitting or nipple 35 throughthe rear wall of the motor for attachment to a conduit leading from anexternal supply source of cryogenic fluid (not shown). The exit end ofthe spiral conduit connects with bores 36 and 37 through the motorhousing to the exterior thereof, in position to communicate with anannular recess 38 of the cutter cage which in turn communicates withbores 39 extending forwardly through the material of the cutter cage tothe rear ends of hubs 28 of the conical cutters.

There is a space 40 between the rear end of each hub 28 and thecorresponding circular surface 41 of the cage through which thecryogenic fluid can flow into communication with the ball bearings 25and then out of the cage through space 42, past gear teeth 31 and out ofthe cage in the space between the forward end 24 of the cage and theconical cutters. Each conical cutter has a passageway 42 extending alongthe axis of rotation of the cutter for communicating with space 40 andleading to a position 44 near the apex of the cutter. Each cutter alsohas several lateral passageways 45, 46, and 47 communicating with themain channel 43 through the central part of the cutter and leading tothe exterior of the cutter. A relatively small passageway 48communicates with the forward end of channel 43 leading to the exteriorat the apex of each cutter.

Each channel 39 passing through the material of the cage is providedwith a branch channel 49 extending from channel 39 to a position nearerthe central axis of the unit from which a narrow passageway or nozzle 50leads to a space 51 formed between the part 30 of each conical cutterand the cage, at the vicinity of the gear teeth 32 so that fluid canflow lengthwise through and past the gear teeth to a central space orvoid 52 between member 33 and the conical cutters. The space 52 is incommunication with the lateral channels, 45, 46 and 47 When electricpower is applied to the motor, the rotor will rotate the conical cuttersby means of the meshing bevel gears 32. Although the cutter cage is freeto rotate relative to the motor housing, it will be subject toconsiderable drag force from the bore hole and the elements in it sothat whatever rotation the cage has will be minor as compared with thespeed of rotation ofthe rotor of the motor.

In its flow cycle, the cryogenic fluid, for example liquid nitrogen,will reach the inlet to the spiral passageway 34 of the motor housingbelow its critical temperature at whichthe liquid becomes gaseous.During the course of its flow through the passage 34 a portion of theliquid fluid may be flashed to the gaseous state by heat rejected fromthe cryo-motor. This gas, together with the remainder of the cryogenicflow through the motor housing arrives at the gear coolant nozzles 50and the cutter nozzles 48 and the lateral passageways where it isexpanded from the coolant system working pressure to ambient-bore holepressure, resulting in its transformation to the gaseous state. Thistransformation makes use of the latent heat of vaporization of thecryogen, thus increasing the cooling capacity at the work surface. Theresultant gas effluent cools and cleans the gear system and the cutters,as well as removing cuttings from the work area.

The very cold gas leaving the cutting area carries with it the cuttings,and as the column of chip-containing gas rises up out of the hole, itpicks up heat rejected from that portion of the bore hole wall alreadycut, freezing the wall and accelerating the gas velocity in proportionto the decrease in gas density.

FIG. 3 shows another form of drilling unit which is the preferredembodiment of this invention. A principal difference from the embodimentof H6. 1 resides in the use of only a single drill bit, which isattached directly to the rotor shaft and without any gears. In otherrespects the embodiment of FIG. 3 does not differ greatly from that ofFIG. I. The electric motor 55 is similar to the motor of FIG. 1 in thatit has a cylindrical housing 56 with a rotor within it having a shaft 57rotatable within the housing by means of ball bearings at the forwardand rear ends, of which only the forward bearing 58 is shown. Thecylindrical housing is somewhat similar to that of FIG. 1 in that itcontains a spiral passageway 59, the inlet end of which is incommunication with an inlet pipe 60 through which the cryogenic fluidenters the passageway. The electrical conduit for driving the motor (notshown) will also pass through this inlet pipe 60 into the motor.

Instead of having a plurality of drill bits as the forward end of therotor shaft, there is provided only a single drill bit 61, this having ashank 62 provided with means for attachment to the forward end of therotor shaft which protrudes from the housing. While any suitableattaching means may be used, the means shown are internal threads 63 onthe shank which thread tightly onto external threads on the shaft. Therear end of the shank substantially meets the ball bearing arrangement58 which is held in place by a suitable retaining ring 64 set into agroove in the housing. A sealing ring 65 helps in keeping dirt anddebris from moving back to the ball bearing.

The drill bit is formed to leave a space 66 between the end of shaft 57and the material of the drill bit. This space 66 communicates with acentral passageway 67 which extends all the way through the rotor shaftalong its longitudinal axis from the rear to the forward end so that therear end of the passageway 67 communicates with conduit 60 and theforward end communicates with space 66. There extends forwardly from thespace 66, along the axis of rotation, but terminating short of theforward end of the drill bit, a passageway or channel 68, and fromthisaxially extending channel there branch out from the sides throughthe drill bit a number of laterally extending passageways 69 leading tothe exterior of the drill bit. Other passageways 70 extend laterallyfrom space 66.

The forward end of the spiral conduit 59 through the motor housingcommunicates with a passageway 71 leading into annular space 72 behindthe bearing 58, so as to communicate with the bearing and thense with anannular space 73 in front of the bearing. The. construction of the sealring 65 is such that fluid in space 73 can be forced through thematerial or convolutions of this seal to the exterior of the drillingunit through a space 74 between the hub of the drill bit and the forwardend of the motor housing.

In operation of the unit of FIG. 3, when the electric power has turnedon the motor, the drill bit fixed to the motor shaft turns with theshaft, and at the same time the cryogenic fluid is forced through theentrance pipe into the motor housing which it divides so that part ofthis coolant flows through the shaft passageway 67 while the other partflows through the spiral passageway 59. That part of the coolant fluidwhich passes through the shaft passageway 67 enters the space 66 fromwhere it divides to exit from the surface of the drill bit through thevarious passageways 68, 69 and 70, thereby cleansing the cuttingsurfaces of the drill and carrying away the cuttings up the bore hole inthe manner described in connection with the embodiment of FIG. 1. Thatpart of the coolant which passes through the spiral passageway 59 flowsfrom space 72 and through the ball bearing thereby cleansing and coolingthe bearing and then out through the seal ring and space 74 to the borehole.

The maintenance of a very cold work area and tool temperature throughthe use of cryogenic cooling, according to this invention, will vastlyincrease the tool life as compared with tools which are notcryogenically cooled.

The positioning of a high-power density motor, as abovedescribed, in theconfined work space of the bore hole, through use of thecryogenic-cooled design permits a greatly improved penetration ratecapability due to higher power available at the work face for a givenhole diameter.

The freezing of the work area by the cold effluent from the cuttersserves to prevent balling or gumming of the cutters in some formations.

The cryogenic fluid by freezing the work surface will cause 'wet gummingformations, such as shale, to become hard and competent, and hence easyto drill. The freezing makes all formations appear to the drill to benearly the same in competence, strength, hardness and drillability,thereby avoiding the necessity for making special provision fordifferent drilling conditions and different kinds of formations.

The heat capacity of the drilling system will, because of the presenceof the very cold cryogen and also the effect of latent heat ofvaporization of the cryogen, keep the drill bit cold, thereby increasingthe life of the bit, and will cause the hole walls to freeze for asignificant distance above the work surface, thus maintaining acompetent hole wall while keeping out water, oil, gas and otherundesirable inflow, and reducing the necessity for frequent casingoperations.

lt has been found that chip removal by gaseous nitrogen effluentaccording to the present invention, is much improved over thatexperienced in conventional air-gas drilling, because the cryogenic gasis heated due to earth temperature as the gas rises up the hole, andthis heating causes an increase in gas velocity while moving up thehole. This increase of velocity improves the chip removal capability asa function of distance up the hole.

It has been found that the cryogenic cooling provided by use of thepresent invention allows a much higher drill rotational speed than inconventional drilling procedures, thereby permitting lower drill bitface pressure than in conventional procedures, which in turn greatlyimproves the directional stability especially in sloping formation andhole wall smoothness, as well as increasing the drilling penetrationrate and the bit life.

What is claimed is:

1. A power-driven, coolable drill unit for boring a hole in earthcomprising a cryo-motor having a rotor and a housing which surroundssaid rotor, a shaft, said rotor being connected to said shaft, and saiddrill bit means fastened at the forward end of said shaft, said drillbit means having a drill face, coolant conduit means for the unitcomprising a passageway for fluid coolant through said motor within thewall of the housing which exits from the housing at the exit means, saidfluid passageway being in the form of a spiral around said rotor,

said passageway having coolant fluid inlet means into said motor andexit means at a more forward part of said motor than the inlet means,said passageway means being in communication with said exit means andpassing through said drill bit means to its exterior, said coolant fluidin said passageway means being substantially a cryogenic liquid which isflashed to its gaseous state adjacent said drill face whereby coolantfluid introduced at the fluid inlet means passes through said motor andsaid drill bit means to the bore hole.

2. Apparatus according to claim 1 wherein said cryogenic liquid which isflashed to its gaseous state adjacent said drill face renders variousearth formations of different character to be of substantially similarcompetence, strength, hardness and drillability relative to the drillbit means.

3. Apparatus according to claim 1 including bearing means in which theshaft is mounted for rotation relative to the housing, in which thepassageway within the wall of the housing is in communication with thebearing means.

4. Apparatus according to claim 1 in which gear means is providedfordrivably attaching the drill bit means to the rotor.

5. Apparatus according to claim 1 in which a cage is attached to theforward end of the motor and the drill bit means is rotatably mounted inbearing means within the cage.

6. Apparatus according to claim 1 in which the coolant conduit meansincludes a fluid passageway within the cage in communication with thepassageway for fluid coolant through the motor and leading to the drillbit means.

7. Apparatus according to claim 1 in which the drill bit means containsconduit means communicating with the fluid passageway within the cage,and leading to the exterior of the drill bit means to expel coolantfluid into the bore hole.

8. Apparatus according to claim 1 in which the bearing means is includedin the coolant conduit means.

9. The method of earth drilling which comprises powering a drill bitmeans by a cryo-motor to which the drill bit means having a drill faceis drivably attached, introducing cryogenic fluid into the motor in theliquid state below the critical temperature of the fluid to absorb heatrejected from the motor, conveying cryogenic fluid out of the motor andthrough the drill bit means in substantially a liquid state, andflashing said cryogenic fluid to its gaseous state adjacent said drillface while ejecting it from the drill bit means.

10. The method according to claim 9 wherein the cryogenic fluid ejectedfrom the drill bit means flashes to a gas adjacent said drill face andacross the work surface, which method includes cooling the bit and worksurface by vaporization, and the existent low temperature, and carryingthe cuttings out of the bore hole with the effluent gas.

11. The method according to claim 9 wherein cryogenic fluid is ejectedfrom the drill bit means and includes freezing the work surface torender various formations of different character to be similar incompetence, strength, hardness and drillability relative to the drillbit.

12. The method according to claim 10 wherein the cryogenic gas is heatedby the surrounding earth as the gas rises up the hole, said gasincreasing in velocity in its upward movement, facilitating chipremoval.

13. The method according to claim 9 wherein the cryomotor has a rotormeans supported by a bearing means, said method including the step ofcooling the bearing means with at least some of the coolant fluidintroduced into the motor.

14. The method according to claim 9 wherein said cryomotor has a housingsurrounding a rotor and said housing has a wall with a passagewaytherein, said method including the step of absorbing heat rejected fromsaid cryo-motor into cryogenic fluid passing through said passageway,said cryogenic fluid passing through said passageway being at least someof the cryogenic fluid introduced into the motor.

15. The method according to claim 14 wherein the cryogenic fluid passingthrough the passageway of the motor housing wall is sent in a spiralpath around the rotor.

16. The method according to claim 14 wherein at least some of thecryogenic fluid introduced into the motor is conveyed to said drill bitmeans throu h said rotor 17. A power-driven coo able drill unit forboring a hole in earth, comprising a cryo-motor having a shaft, a rotormounted on the shaft and a housing which surrounds the rotor, saidhousing having a wall, drill bit means drivably attached to the shaft atthe forward end of the motor, and coolant conduit means for the unitcomprising a passageway for fluid coolant through the motor andincluding at least a spiral passage within the wall of the housing, saidpassageway having coolant fluid inlet means into the motor and exitmeans at a more forward part of the motor than the inlet means, andpassageway means in communication with said exit means and passingthrough the drill bit means to its exterior, whereby coolant fluidintroduced at fluid inlet means passes through the motor and the drillbit means to the bore hole.

18. Apparatus according to claim 17 wherein a cage is attached to theforward end of the motor and the drill bit means is rotatably mounted inbearing means within the cage, a gear means is mounted on the shaft fordriving the drill bit means, and the coolant conduit means includes afluid passageway within the cage in communication with the passagewayfor fluid coolant through the motor and leading to the drill bit means.

19. Apparatus according to claim 18 wherein the drill bit means containsconduit means communicating with the fluid passageway within the cageand leading to the exterior of the drill bit means to expel coolantfluid into the bore hole.

20. Apparatus according to claim 19 wherein the bearing means isincluded in the coolant conduit means.

1. A power-driven, coolable drill unit for boring a hole in earthcomprising a cryo-motor having a rotor and a housing which surroundssaid rotor, a shaft, said rotor being connected to said shaft, and saiddrill bit means fastened at the forward end of said shaft, said drillbit means having a drill face, coolant conduit means for the unitcomprising a passageway for fluid coolant through said motor within thewall of the housing which exits from the housing at the exit means, saidfluid passageway being in the form of a spiral around said rotor, saidpassageway having coolant fluid inlet means into said motor and exitmeans at a more forward part of said motor than the inlet means, saidpassageway means being in communication with said exit means and passingthrough said drill bit means to its exterior, said coolant fluid in saidpassageway means being substantially a cryogenic liquid which is flashedto its gaseous state adjacent said drill face whereby coolant fluidintroduced at the fluid inlet means passes through said motor and saiddrill bit means to the bore hole.
 2. Apparatus according to claim 1wherein said cryogenic liquid which is flashed to its gaseous stateadjacent said drill face renders various earth formations of differentcharacter to be of substantially similar competence, strength, hardnessand drillability relative to the drill bit means.
 3. Apparatus accordingto claim 1 including bearing means in which the shaft is mounted forrotation relative to the housing, in which the passageway within thewall of the housing is in communication with the bearing means. 4.Apparatus according to claim 1 in which gear means is provided fordrivably attaching the drill bit means to the rotor.
 5. Apparatusaccording to claim 1 in which a cage is attached to the forward end ofthe motor and the drill bit means is rotatably mounted in bearing meanswithin the cage.
 6. Apparatus according to claim 1 in which the coolantconduit means includes a fluid passageway within the cage incommunication with the passageway for fluid coolant through the motorand leading to the drill bit means.
 7. Apparatus according to claim 1 inwhich the drill bit means contains conduit means communicating with thefluid passageway wIthin the cage, and leading to the exterior of thedrill bit means to expel coolant fluid into the bore hole.
 8. Apparatusaccording to claim 1 in which the bearing means is included in thecoolant conduit means.
 9. The method of earth drilling which comprisespowering a drill bit means by a cryo-motor to which the drill bit meanshaving a drill face is drivably attached, introducing cryogenic fluidinto the motor in the liquid state below the critical temperature of thefluid to absorb heat rejected from the motor, conveying cryogenic fluidout of the motor and through the drill bit means in substantially aliquid state, and flashing said cryogenic fluid to its gaseous stateadjacent said drill face while ejecting it from the drill bit means. 10.The method according to claim 9 wherein the cryogenic fluid ejected fromthe drill bit means flashes to a gas adjacent said drill face and acrossthe work surface, which method includes cooling the bit and work surfaceby vaporization, and the existent low temperature, and carrying thecuttings out of the bore hole with the effluent gas.
 11. The methodaccording to claim 9 wherein cryogenic fluid is ejected from the drillbit means and includes freezing the work surface to render variousformations of different character to be similar in competence, strength,hardness and drillability relative to the drill bit.
 12. The methodaccording to claim 10 wherein the cryogenic gas is heated by thesurrounding earth as the gas rises up the hole, said gas increasing invelocity in its upward movement, facilitating chip removal.
 13. Themethod according to claim 9 wherein the cryo-motor has a rotor meanssupported by a bearing means, said method including the step of coolingthe bearing means with at least some of the coolant fluid introducedinto the motor.
 14. The method according to claim 9 wherein saidcryo-motor has a housing surrounding a rotor and said housing has a wallwith a passageway therein, said method including the step of absorbingheat rejected from said cryo-motor into cryogenic fluid passing throughsaid passageway, said cryogenic fluid passing through said passagewaybeing at least some of the cryogenic fluid introduced into the motor.15. The method according to claim 14 wherein the cryogenic fluid passingthrough the passageway of the motor housing wall is sent in a spiralpath around the rotor.
 16. The method according to claim 14 wherein atleast some of the cryogenic fluid introduced into the motor is conveyedto said drill bit means through said rotor.
 17. A power-driven coolabledrill unit for boring a hole in earth, comprising a cryo-motor having ashaft, a rotor mounted on the shaft and a housing which surrounds therotor, said housing having a wall, drill bit means drivably attached tothe shaft at the forward end of the motor, and coolant conduit means forthe unit comprising a passageway for fluid coolant through the motor andincluding at least a spiral passage within the wall of the housing, saidpassageway having coolant fluid inlet means into the motor and exitmeans at a more forward part of the motor than the inlet means, andpassageway means in communication with said exit means and passingthrough the drill bit means to its exterior, whereby coolant fluidintroduced at fluid inlet means passes through the motor and the drillbit means to the bore hole.
 18. Apparatus according to claim 17 whereina cage is attached to the forward end of the motor and the drill bitmeans is rotatably mounted in bearing means within the cage, a gearmeans is mounted on the shaft for driving the drill bit means, and thecoolant conduit means includes a fluid passageway within the cage incommunication with the passageway for fluid coolant through the motorand leading to the drill bit means.
 19. Apparatus according to claim 18wherein the drill bit means contains conduit means communicating withthe fluid passageway within the cage and leading to the exterior of thedrill bit means to expel coolant fluid into the bore hole.
 20. Apparatusaccording to claim 19 wherein the bearing means is included in thecoolant conduit means.